Isoforms of the inositol 1,4,5-trisphosphate receptor are expressed in bovine oocytes and ovaries: The type-1 isoform is down-regulated by fertilization and by injection of adenophostin A

Mammalian fertilization is characterized by the presence of long-lasting intracellular calcium ([Ca2+](i)) oscillations that are required to induce oocyte activation. One of the Ca2+ channels that may mediate this Ca2+ release is the inositol 1,4,5-trisphosphate receptor (IP3R). Three isoforms of the receptor have been described, but their expression in oocytes and possible roles in mammalian fertilization are not well known. Using isoform-specific antibodies against IP3R types 1, 2, and 3 and Western analysis, we determined the isoforms that are expressed in bovine metaphase II oocytes and ovaries, In oocytes, all isoforms are expressed, but type 1 is present in overwhelmingly larger amounts and is likely responsible for the majority of Ca2+ release at fertilization. In ovarian microsomes, all three isoforms appear well expressed, suggesting the participation of all IP3R isoforms in ovarian Ca2+ signaling. We then investigated whether the reported cessation/reduction in amplitude of fertilization-associated [Ca2+](i) oscillations, which is observed as pronuclear formation approaches, corresponded with down-regulation of the IP3R-1 isoform. Fertilization resulted in approximately 40% reduction in the amount of receptor by 16 h postinsemination. In addition, injection of adenophostin A, a potent IP3R agonist that elicits high-frequency [Ca2+](i) oscillations in mammalian oocytes, induced similar reduction in receptor numbers. Together, these data show that 1) the three IP3R isoforms are expressed in bovine oocytes; 2) IP3R-1 is likely to mediate most of the Ca2+ release during fertilization; 3) its down-regulation may explain the decline in amplitude of sperm-induced [Ca2+](i) rises as fertilization progresses toward pronuclear formation; and 4) agonists of the IP3R induce down-regulation of the type-1 receptor in oocytes similar to that evoked by fertilization.